Alzheimer's disease is a devastating neurodegenerative disease that afflicts over 4 million people in the United States. Extracellular senile plaques, one of the defining pathological features of the disease, consist of a proteinaceous amyloid deposit surrounded by degenerating neurites. The predominant protein component of the amyloid deposit is beta-amyloid (Abeta), a 4 kDa peptide derived from a much larger transmembrane precursor protein, deposited as insoluble fibrils of approximately 10 nm diameter and cross-beta structural motif. In vitro, genetic-linkage, and transgenic animal studies link aggregation of Abeta to cellular toxicity; thus, Abeta aggregation likely plays an essential role in the onset and/or progression of Alzheimer's pathology. The assembly state(s) responsible for toxicity has not yet been definitively identified, and the mechanism by which it exerts its toxic effect remains unknown. One emerging hypothesis is that Abeta perturbs lipid bilayer structure and dynamics, causing malfunctioning of membrane-embedded proteins responsible for ion flux regulation, signal transduction, adhesion, and other essential functions. The overall objective of the proposed project is to define the connection between Abeta assembled state, Abeta-membrane perturbations, and Abeta cellular toxicity in vitro. In previous work biophysical experimental techniques and kinetic modelling were employed to characterize the kinetics of assembly from monomeric Abeta to fibril. As the first specific aim of the proposed work, these results will be extended to encompass temperature dependence, water activity effects, and heterogeneous association between soluble and precipitated Abeta species. Preliminary work indicates that Abeta decreases membrane fluidity, in a manner that depends strongly on the Abeta assembly state. This observation motivates the second objective of the proposed work, wherein Abeta-mediated membrane perturbation will be further evaluated to define changes in fluidity and lipid hydration, as a function of Abeta assembly state. Compounds possessing inhibitory activity against Abeta toxicity in vitro have been described in the literature but their mechanism of action is poorly understood. In the third specific aim of this project, three representative compounds (a peptide, a sulfonated compound, and an anthracycline derivative), belonging to distinct chemical families, will be tested for their ability to disrupt Abeta assembly. Additionally, the compounds' effectiveness at modulating Abeta-mediated membrane perturbation and protecting cells from Abeta toxicity will be assessed. Systematic evaluation of these modulatory compounds should provide key information leading to the identification of the Abeta assembly state responsible for toxicity, and elucidation of one possible mechanism of toxicity. The results should also lead to improved design of inhibitors of Abeta aggregation and toxicity.